DE 103 41 027 A1, which is incorporated herein by reference, discloses a method for inter-vehicle distance control (ACC) for a motor vehicle, which method can be carried out in or with a two-circuit ABS/ESP brake control device which is commonly used with a pump. In order to meter the hydraulic brake pressure, an electrically actuable hydraulic isolating valve which is controlled in an analog manner is used, said isolating valve permitting, in contrast to control with an inlet valve which is actuated in an analog manner, pressure control throughout the brake circuits. The hydraulic valves used for pressure control are generally so-called analog/digital valves (A/D valves) which substantially correspond to conventional electromagnetic (solenoid) switching valves, but said analog/digital valves being actuated by means of a pulse-width-modulated current (PWM) in such a way that the valve tappet assumes a floating position. Pressure control is possible in this way provided that the current is actuated in a sufficiently accurate and reproducible manner. This is generally possible without major problems if the pressure to be adjusted can be adjusted by means of a pressure sensor in conjunction with a control loop. It is considerably more difficult to carry out a corresponding analog control method without a pressure sensor in the pressure circuit to be controlled. In this case, stored valve characteristic curves are usually used, these permitting, in conjunction with control methods which are known per se, the pressure to be set in the wheel without additional wheel pressure sensors.
The valve characteristic curve mentioned above therefore permits the pressure to be set by the valve in accordance with a pressure requirement which is applied to the controller, as an input variable. In this connection, DE 103 41 027 A1 mentions the problem that the dead space of the hydraulics, for example the air gap between the brake lining and brake disk, can lead to uncomfortable control during inter-vehicle distance control operation. As a solution to this problem, said application proposes applying an offset to the calibration characteristic curve.
DE 10 2004 008 796 A1, which is incorporated herein by reference, discloses a method and an apparatus for controlling a hydraulic valve of a hydraulic system, in which method an opposing force is generated at a solenoid valve, which is actuated by a brake control system, in order to damp noise by the valve coil being electrically excited at a suitable frequency. The method relates to pressure control methods in which the pressure is increased and reduced by means of valves which are switched in accordance with a pulse sequence.